An optical fiber is usually covered by a sheath of plastic that serves two functions:
protecting the fiber against any external abrasion so as to retain its mechanical strength (it should be recalled that the mechanical strength of a fiber is limited by microcracks in its surface, and the greater the size of the microcracks, the less the mechanical strength); and
protecting the fiber against sharp bends in the cable in which it is inserted, since sharp bends could degrade the optical properties of the fiber.
In certain applications, such as wire-guiding or various applications in industrial environments, the fiber is in the presence of a hostile atmosphere: water vapor, water, or corrosive liquids, e.g. oil and hydrogen.
Under the combined effects of such an environment and of mechanical stress, it is observed that the plastic covering is inadequate, microcracks grow and reduce the mechanical strength of the fibers. This phenomenon is known as corrosion under stress. In addition, if hydrogen diffusion is taking place, it is observed that the glass itself is degraded as are the optical properties of the fiber.
For all of the above reasons, it turns out to be essential to provide a hermetic coating of carbon on the fiber for the purpose of preventing the corrosive environment diffusing into contact with the surface of the fiber.
Such a coating can be made by pyrolysis of a gaseous hydrocarbon flowing through a reactor placed in an oven heated to about 1000.degree. C. by the Joule effect. The fiber then runs longitudinally through the reactor.
The method is mentioned in an article published in "Journal of Lightwave Technology", Vol. 6, No. 2, February 1988, pp. 240-241 entitled "Recent developments in hermetically coated optical fiber" by K. E. Lu et al.
It is observed that implementing the above method causes a carbon deposit to be obtained that does not adhere well and that is incapable of providing adequate protection to the fiber.
It turns out that an essential condition for obtaining a carbon deposit of good quality is that the silica fiber should be at a sufficiently high temperature on entering the reactor, which temperature should be about 300.degree. C. greater than the minimum pyrolysis temperature.
This condition could possibly be obtained if the reactor were located very close to the outlet of the fiber-drawing oven and if the fiber was running fast enough to avoid cooling down. To make this possible, the fiber-drawing speed must be greater than 150 meters per minute.
Since this fiber-drawing speed may be too high for the optical qualities of the fiber to be guaranteed, it is appropriate at lower speeds to interpose a heater device between the fiber-drawing oven and the reactor.
French patent FR-A-90 02 197 has already proposed solving this problem by means of a Joule effect oven. Such an oven suffers from drawbacks: it lacks efficiency because a silica fiber is transparent to infrared radiation; and it would need to be very long to obtain adequate heating.
An object of the present invention is to implement a method and apparatus for heating the fiber that is efficient and compact, suitable for raising the fiber to a temperature that enables a good quality carbon deposit to be obtained, regardless of the fiber-drawing speed; thereby making it possible to optimize both the carbon deposit and the optical characteristics of the fiber simultaneously.